Undisturbed ecosystems maintain themselves by cycling nutrients and other inputs, like water and energy, up through food webs. As discussed in Unit 4, “Ecosystems,” these cycles are closed loops to a large extent. Substances change form as they move through ecosystems, but they are not destroyed or removed from the system.

Agriculture is fundamentally different from undisturbed ecosystems because harvesting crops removes material from the system. The product that can be harvested from an agricultural system, which is called its yield, represents a loss of materials such as water and nutrients from the system. Farmers can increase yields by adding energy and materials, by increasing the efficiency of energy conversion and allocation to the harvested product, or by reducing losses that occur during the growing process.

Agricultural yields have risen steadily throughout the history of human cultivation, with particularly steep increases throughout the 20th century. From 1961 through 1999, the FAO's aggregate crop production index increased at an average rate of 2.3 percent per year and world crop production per capita rose at an average annual rate of 0.6 percent (footnote 6). Global production of major cereal crops more than doubled during this period (Fig. 6).

Productivity in agriculture is a measurement of farmers' total output per unit of land. If the gains shown in Figure 6 had come simply from bringing twice as much land under cultivation, they would not automatically signal that productivity was rising as long as farmers were using the same amount of inputs per acre. However, agriculture has become much more productive over time. In many parts of the world, modern farmers get far more product from each unit of land than their predecessors thanks to intensification—using more technological inputs per acre. In areas where such inputs are not available, such as Africa, output rates remain far below world averages.

Radical changes in agricultural inputs over the past century made this increase possible. Land and labor inputs have fallen drastically in industrialized countries, but technological advances such as large-scale irrigation, synthetic fertilizers, pesticides and herbicides, and capital investment (in the form of mechanization) have increased sharply. Scientific advances such as development of higher-yielding crop varieties have also contributed to increased productivity.

The most significant way in which scientists have produced bigger yields is by modifying plants so that they devote a larger proportion of their physical structures to producing biomass that is usable for food. This process is referred to as increasing the harvest index (the ratio of harvested biomass to total biomass). For example, growing deep root systems protects wild plants against drought, but this allocation strategy limits the amount of plant biomass available to make leaves, so they have fewer sugars from photosynthesis available to make seeds. Plant breeders selecting for higher-yielding varieties might try to increase the harvest index by including plants that produce fewer roots and more seeds, or by developing dwarf or semi-dwarf strains. Figure 7 shows several modifications that have increased rice yields.

This approach was an important component of the "Green Revolution"—a 30-year transformation of agriculture in developing regions that started in the 1940s, when private foundations and national governments joined forces to distribute high-yielding crop varieties, synthetic fertilizer, irrigation, and pesticides to subsistence farmers in Asia and Latin America. By introducing semi-dwarf varieties of wheat and rice, researchers increased the crops' harvest indexes and reduced the problem of lodging (falling over before harvest due to excessive growth). This shift made it possible for farmers to apply higher levels of chemical fertilizers so that plants would photosynthesize at increased rates and produce more biomass. Scientists also developed these new strains to make them easier to harvest, more durable during transport, and longer-lasting in storage.

The Green Revolution helped world food production to increase at a rate faster than population growth from 1950 onward. However, these increases relied on synthetic fertilizer and irrigation because green revolution plant varieties were designed to produce high yields when supplied with high inputs of nitrogen and water. In other words, they were not inherently high-yielding plants (i.e., they were not able to use resources more efficiently than traditional varieties) and likely would have done worse under "natural" conditions. Many varieties were highly susceptible to pests and diseases, so they also required heavy use of pesticides to thrive. Because the new plants were short, they were more susceptible to competition from weeds, so farmers also had to use herbicides to raise them.

As we will see in the next section, this strategy generated further complications for human health, non-target species, and the environment in the regions where it was applied. Conversely, because Green Revolution agriculture is capital-intensive and requires well-developed infrastructure systems for functions such as delivering irrigation water, it essentially bypassed sub-Saharan Africa.